联合Argo浮标、卫星测高和GRACE数据研究海平面变化

卫星测高、GRACE、Argo等数据为监测海平面变化提供了丰富的观测数据,利用Argo数据计算的比容海平面变化,可以更加深入地理解卫星测高以及卫星重力获得的海平面变化。利用2004年1月至2010年12月间Argo浮标采集的温度和盐度数据,通过数值积分方法计算了65°S~65°N间的比容海平面异常,并通过最小二乘拟合得到比容海平面变化的长期趋势为0.63±0.45 mm/a,与Llovel得到的结果吻合较好。利用卫星测高数据得到该时间段内海平面变化趋势为2.52±0.71 mm/a,GRACE反演得到的海水质量变化引起的海平面趋势为1.84±0.13mm/a,结果表明海水质量变化成为引起海平面变化的主要因素。最后对联合卫星测高、GRACE得到比容海平面变化与相应Argo浮标数据计算结果的空间分布特征进行了比较。     

联合卫星测高、GRACE与温盐数据分析红海海平面变化季节性特征

本文利用卫星测高、GRACE与温盐数据监测2003-2014年红海海平面变化,并分析了蒸发降水以及亚丁湾-红海质量交换对红海质量变化的影响。红海地区单一的温盐数据存在覆盖不全或质量不佳的问题,综合CORA、SODA与ORAS4温盐数据估算结果得到平均比容海平面变化,以改善比容信号的精度。针对GRACE数据处理过程中截断与空间平滑滤波引起的泄漏误差,提出改进尺度因子纠正泄漏误差,利用卫星测高数据进行模拟实验验证了改进尺度因子的有效性。利用传统尺度因子和改进尺度因子反演的红海质量变化周年振幅分别为16.1±1.3 cm和20.5±1.7 cm,利用卫星测高和温盐数据估算的质量变化周年振幅为20.2±1.0 cm,表明改进尺度因子可有效减小泄漏误差的影响,改善GRACE模型反演红海质量变化的精度。卫星测高、GRACE卫星重力数据以及平均温盐数据具有较好的一致性,联合GRACE和温盐数据估算的红海综合海平面变化周年振幅为16.6±1.7 cm,与卫星测高估算的总海平面变化周年振幅(16.2±0.9 cm)基本一致,表明多源数据可构成完整的红海海平面监测手段。相比于降水-蒸发作用,红海质量变化受红海与亚丁湾的海水质量交换的影响更为显著,其主导了红海质量的季节性变化。     

Seasonal global mean sea level change from satellite altimeter, GRACE, and geophysical models

We estimate seasonal global mean sea level changes using different data resources, including sea level anomalies from satellite radar altimetry, ocean temperature and salinity from the World Ocean Atlas 2001, time-variable gravity observations from the Gravity Recovery and Climate Experiment (GRACE) mission, and terrestrial water storage and atmospheric water vapor changes from the NASA global land data assimilation system and National Centers for Environmental Prediction reanalysis atmospheric model. The results from all estimates are consistent in amplitude and phase at the annual period, in some cases with remarkably good agreement. The results provide a good measure of average annual variation of water stored within atmospheric, land, and ocean reservoirs. We examine how varied treatments of degree-2 and degree-1 spherical harmonics from GRACE, laser ranging, and Earth rotation variations affect GRACE mean sea level change estimates. We also show that correcting the standard equilibrium ocean pole tide correction for mass conservation is needed when using satellite altimeter data in global mean sea level studies. These encouraging results indicate that is reasonable to consider estimating longer-term time series of water storage in these reservoirs, as a way of tracking climate change.     

联合卫星重力、卫星测高和海洋资料研究全球海平面变化

利用GRACE、卫星测高和海洋实测温盐数据,探讨了2003~2012年间全球海平面、比容海平面和海水质量等的变化特征,并讨论了南极冰盖和格陵兰冰盖消融对全球海平面变化的影响。全球海平面整体呈上升趋势,上升速度为2.72±0.07 mm/a,且存在显著的空间分布特征。全球海平面、比容海平面和海水质量等的变化还具有显著的季节性特征,其中全球海平面变化的年周期振幅为4.6±0.3 mm。使用经验正交函数分析(EOF)得到全球海平面和比容海平面的季节性变化在南北半球存在显著的差异,但海水质量季节性变化不存在这种差异。南极冰盖和格陵兰冰盖的消融速率分别为-75.7±12.3 Gt/a和-124.1±2.9 Gt/a,对海平面的长期趋势项贡献分别为0.21±0.03 mm/a和0.34±0.01 mm/a,仅占全球海水质量增加速度1.80±0.10 mm/a的12%和19%,总计占31%,因此,两极冰盖质量消融并不是2003-2012年间海水质量增加的最主要因素。     

ENSO影响下的西太平洋地区海陆水储量变化分析

利用2005-01-2016-12的GRACE卫星数据,结合测高、ARGO（array for real-time geostrophic oceanography）、TRMM（tropical rainfall measurement mission）降水数据和MEI（multivariate ENSO index）,从海平面、海水质量、海水比容和陆地水储量的非季节性变化分析了厄尔尼诺-南方涛动（El Ni?o/Southern Oscillation,ENSO）对西太平洋及其沿岸地区的影响。在GRACE数据处理过程中考虑了地震构造信号的影响,并采用正演模型法来恢复时变信号。结果表明,西太平洋地区平均海平面（western Pacific mean sea level,WPMSL）变化对ENSO的响应表现为:厄尔尼诺期间海平面下降,拉尼娜期间海平面上升;尤其在2014-2016年厄尔尼诺期间,WPMSL下降了近26.2 mm。比容变化为影响WPMSL非季节性变化的主导因素。澳大利亚和中南半岛水储量异常在年际变化上也较好响应了ENSO,与MEI的相关系数分别为-0.61（滞后3个月）和-0.65（滞后8个月）。长江流域对ENSO的响应主要在中下游区域,2014-2016年厄尔尼诺造成了该时期长江流域水储量达到近几年的极大值。     

基于多源数据的全球比容海平面变化研究

利用2005—2009年共60个月的卫星测高、GRACE数据计算全球66°S~66°N比容海平面变化,同时利用Argo数据计算得到该区域的比容海平面变化,结果可知比容海平面具有明显的周年变化。比较不同方法得到的比容海平面变化发现二者在整体趋势上较为一致,但局部也存在着差异。联合卫星测高、GRACE和利用Argo数据得到2005—2009年比容海平面变化振幅分别为10.5 mm和4.3 mm,长期变化趋势分别为1.63 mm/a和0.32mm/a。     

Decorrelated GRACE time-variable gravity solutions by GFZ,and their validation using a hydrological model

We have analyzed recent gravity recovery and climate experiment (GRACE) RL04 monthly gravity solutions, using a new decorrelating post-processing approach. We find very good agreement with mass anomalies derived from a global hydrological model. The post-processed GRACE solutions exhibit only little amplitude damping and an almost negligible phase shift and period distortion for relevant hydrological basins. Furthermore, these post-processed GRACE solutions have been inspected in terms of data fit with respect to the original inter-satellite ranging and to SLR and GPS observations. This kind of comparison is new. We find variations of the data fit due to solution post-processing only within very narrow limits. This confirms our suspicion that GRACE data do not firmly ‘pinpoint’ the standard unconstrained solutions. Regarding the original Kusche (J Geod 81:733–749, 2007) decorrelation and smoothing method, a simplified (order-convolution) approach has been developed. This simplified approach allows to realize a higher resolution—as necessary, e.g., for generating computed GRACE observations—and needs far less coefficients to be stored.     

联合时变重力数据与测高数据反演全球海平面变化及其分量贡献

理解全球海平面变化具有十分重要的意义,它间接反映了地球系统中气候性相关因素的变化。本文基于一组海平面指纹和比容经验正交函数,联合时变重力数据和卫星测高数据反演了2002年4月至2020年2月的全球海平面变化,将全球海平面变化分解成南极冰盖融化、格陵兰冰盖融化、陆地冰川融化、陆地水储量变化、冰川均衡调整和海水比容效应这6个分量的贡献。联合反演结果显示,全球平均比容海平面变化为1.08±0.05 mm/a,与相关文献的结果相吻合。研究发现,联合测高数据和时变重力数据的反演方法能够一定程度上减弱GRACE Follow-On卫星时期海水质量变化被低估的现象。本文利用联合反演的结果研究了区域海平面变化,在大部分近海区域反演效果较好,这表明该方法可用于区域海平面变化的研究。     

Statistically optimal estimation of Greenland Ice Sheet mass variations from GRACE monthly solutions using an improved mascon approach

We present an improved mascon approach to transform monthly spherical harmonic solutions based on GRACE satellite data into mass anomaly estimates in Greenland. The GRACE-based spherical harmonic coefficients are used to synthesize gravity anomalies at satellite altitude, which are then inverted into mass anomalies per mascon. The limited spectral content of the gravity anomalies is properly accounted for by applying a low-pass filter as part of the inversion procedure to make the functional model spectrally consistent with the data. The full error covariance matrices of the monthly GRACE solutions are properly propagated using the law of covariance propagation. Using numerical experiments, we demonstrate the importance of a proper data weighting and of the spectral consistency between functional model and data. The developed methodology is applied to process real GRACE level-2 data (CSR RL05). The obtained mass anomaly estimates are integrated over five drainage systems, as well as over entire Greenland. We find that the statistically optimal data weighting reduces random noise by 35–69%, depending on the drainage system. The obtained mass anomaly time-series are de-trended to eliminate the contribution of ice discharge and are compared with de-trended surface mass balance (SMB) time-series computed with the Regional Atmospheric Climate Model (RACMO 2.3). We show that when using a statistically optimal data weighting in GRACE data processing, the discrepancies between GRACE-based estimates of SMB and modelled SMB are reduced by 24–47%.     

Evaluation and validation of mascon recovery using GRACE KBRR data with independent mass flux estimates in the Mississippi Basin

The direct recovery of surface mass anomalies using GRACE KBRR data processed in regional solutions provides mass variation estimates with 10-day temporal resolution. The approach undertaken herein uses a tailored orbit estimation strategy based solely on the KBRR data and directly estimates mass anomalies from the GRACE data. We introduce a set of temporal and spatial correlation constraints to enable high resolution mass flux estimates. The Mississippi Basin, with its well understood surface hydrological modelling available from the Global Land Data Assimilation System (GLDAS), which uses advanced land surface modeling and data assimilation techniques, and a wealth of groundwater data, provides an opportunity to quantitatively compare GRACE estimates of the mass flux in the entire hydrological column with those available from independent and reliable sources. Evaluating GRACE’s performance is dependent on the accuracy ascribed to the hydrological information, which in and of itself is a complex challenge (Rodell in Hydrogeol J, doi:, 2007). Nevertheless, the Mississippi Basin is one of the few regions having a large hydrological signal that can support a meaningful GRACE comparison on the spatial scale resolved by GRACE. The isolation of the hydrological signal is dependent on the adequacy of the forward mass flux modeling for tides and atmospheric pressure variations. While these models have non-uniform global performance they are excellent in the Mississippi Basin. Through comparisons with the independent hydrology, we evaluate the effect on the solution of changing correlation times and distances in the constraints, altering the parameter recovery for areas external to the Mississippi Basin, and changing the relative strength of the constraints with respect to the KBRR data. The accuracy and stability of the mascon solutions are thereby assessed, especially with regard to the constraints used to stabilize the solution. We show that the mass anomalies, as represented by surface layer of water within regional cells have accuracy estimates of ±2–3 cm on par with the best hydrological estimates and consistent with our accuracy estimates for GRACE mass anomaly estimates. These solutions are shown to be very stable, especially for the recovery of semi-annual and longer period trends, where for example, the phase agreement for the dominant annual signal agrees at the 10-day level of resolution provided by GRACE. This validation confirms that mascons provide critical environmental data records for a wide range of applications including monitoring ground water mass changes.     

GRACE-derived surface water mass anomalies by energy integral approach: application to continental hydrology

We propose an unconstrained approach to recover regional time-variations of surface mass anomalies using Level-1 Gravity Recovery and Climate Experiment (GRACE) orbit observations, for reaching spatial resolutions of a few hundreds of kilometers. Potential differences between the twin GRACE vehicles are determined along short satellite tracks using the energy integral method (i.e., integration of orbit parameters vs. time) in a quasi-inertial terrestrial reference frame. Potential differences residuals corresponding mainly to changes in continental hydrology are then obtained after removing the gravitational effects of the known geophysical phenomena that are mainly the static part of the Earth’s gravity field and time-varying contributions to gravity (Sun, Moon, planets, atmosphere, ocean, tides, variations of Earth’s rotation axis) through ad hoc models. Regional surface mass anomalies are restored from potential difference anomalies of 10 to 30-day orbits onto 1^◦ continental grids by regularization techniques based on singular value decomposition. Error budget analysis has been made by considering the important effects of spectrum truncation, the time length of observation (or spatial coverage of the data to invert) and for different levels of noise.     

Annual variations in water storage and precipitation in the Amazon Basin

We combine satellite gravity data from the gravity recovery and climate experiment (GRACE) and precipitation measurements from the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center’s (CPC) Merged Analysis of Precipitation (CMAP) and the Tropical Rainfall Measuring Mission (TRMM), over the period from mid-2002 to mid-2006, to investigate the relative importance of sink (runoff and evaporation) and source (precipitation) terms in the hydrological balance of the Amazon Basin. When linear and quadratic terms are removed, the time-series of land water storage variations estimated from GRACE exhibits a dominant annual signal of 250 mm peak-to-peak, which is equivalent to a water volume change of ~1,800 km³. A comparison of this trend with accumulated (i.e., integrated) precipitation shows excellent agreement and no evidence of basin saturation. The agreement indicates that the net runoff and evaporation contributes significantly less than precipitation to the annual hydrological mass balance. Indeed, raw residuals between the de-trended water storage and precipitation anomalies range from ±40 mm. This range is consistent with stream-flow measurements from the region, although the latter are characterized by a stronger annual signal than our residuals, suggesting that runoff and evaporation may act to partially cancel each other.     

Ice mass change in Greenland and Antarctica between 1993 and 2013 from satellite gravity measurements

We construct long-term time series of Greenland and Antarctic ice sheet mass change from satellite gravity measurements. A statistical reconstruction approach is developed based on a principal component analysis (PCA) to combine high-resolution spatial modes from the Gravity Recovery and Climate Experiment (GRACE) mission with the gravity information from conventional satellite tracking data. Uncertainties of this reconstruction are rigorously assessed; they include temporal limitations for short GRACE measurements, spatial limitations for the low-resolution conventional tracking data measurements, and limitations of the estimated statistical relationships between low- and high-degree potential coefficients reflected in the PCA modes. Trends of mass variations in Greenland and Antarctica are assessed against a number of previous studies. The resulting time series for Greenland show a higher rate of mass loss than other methods before 2000, while the Antarctic ice sheet appears heavily influenced by interannual variations.     

Vertical deformations from homogeneously processed GRACE and global GPS long-term series

Temporal variations in the geographic distribution of surface mass cause surface displacements. Surface displacements derived from GRACE gravity field coefficient time series also should be observed in GPS coordinate time series, if both time series are sufficiently free of systematic errors. A successful validation can be an important contribution to climate change research, as the biggest contributors to mass variability in the system Earth include the movement of oceanic, atmospheric, and continental water and ice. In our analysis, we find that if the signals are larger than their precision, both geodetic sensor systems see common signals for almost all the 115 stations surveyed. Almost 80% of the stations have their signal WRMS decreased, when we subtract monthly GRACE surface displacements from those observed by GPS data. Almost all other stations are on ocean islands or small peninsulas, where the physically expected loading signals are very small. For a fair comparison, the data (79 months from September 2002 to April 2009) had to be treated appropriately: the GPS data were completely reprocessed with state-of-the-art models. We used an objective cluster analysis to identify and eliminate stations, where local effects or technical artifacts dominated the signals. In addition, it was necessary for both sets of results to be expressed in equivalent reference frames, meaning that net translations between the GPS and GRACE data sets had to be treated adequately. These data sets are then compared and statistically analyzed: we determine the stability (precision) of GRACE-derived, monthly vertical deformation data to be ~1.2 mm, using the data from three GRACE processing centers. We statistically analyze the mean annual signals, computed from the GPS and GRACE series. There is a detailed discussion of the results for five overall representative stations, in order to help the reader to link the displayed criteria of similarity to real data. A series of tests were performed with the goal of explaining the remaining GPS–GRACE residuals.     

联合卫星测高、GRACE、海洋和气象资料研究南海海水质量变化

首先利用重力恢复与气候实验（gravity recovery and climate experiment,GRACE）卫星重力、卫星测高和海洋温盐数据分析了2003-2012年间南海海水质量的变化特征,进而结合海洋和气象资料探讨了厄尔尼诺和南方涛动（El Ni?o-Southern Oscillation,ENSO）、净淡水通量、海水体积输送和陆地径流在此期间对南中国海海水质量变化的影响。研究结果表明,南海海水质量变化主要受海面净淡水通量和海水体积输送的联合调制影响,周边陆地径流对其影响有限。南海海水质量季节性变化显著,且具有明显的长期增加趋势;ENSO通过改变降水和黑潮自吕宋海峡流入南海的水量影响南海海水质量,使得南海海水质量存在着显著的具有ENSO特征的年际变化。     

联合GRACE与GPS比较山西省垂向地表形变

利用2009-05~2010-12共20 mon的GRACE时变重力场反演了山西省境内陆地水储量变化造成的地表垂直位移,并与同期的GPS高程残差进行了比较。结果显示,对大部分CORS站而言,GRACE反演的水文地壳垂向形变与GPS高程残差时间序变化量均在-6~6 mm之间;由二者提取的年周期信号平均振幅均为2.5 mm左右,但是相位存在较大差异。由于GPS数据处理过程未考虑周日、半周日大气潮汐,高阶电离层,及非潮汐海潮的影响,这三项误差会传播到测站坐标的时间序列,对GPS周年信号产生影响,所以可以认为GRACE与GPS结果的差异更多的是来自GPS数据处理的误差。     

中国近海及邻近海域高精度高分辨率海面高的确定

利用新的海潮模型(NAO99B)更为合理的共线方法和交叠平差技术,有效改善了ERM数据的径向轨道误差和时变海面高的影响,提高了浅海海域测高数据的测高精度。     

Black sea annual and inter-annual water mass variations from space

This study evaluates the performance of two widely used GRACE solutions (CNES/GRGS RL02 and CSR RL04) in deriving annual and inter-annual water mass variations in the Black Sea for the period 2003–2007. It is demonstrated that the GRACE derived water mass variations in the Black Sea are heavily influenced by the leakage of hydrological signals from the surrounding land. After applying the corresponding correction, we found a good agreement with water mass variations derived from steric-corrected satellite altimetry observations. Both GRACE and altimetry show significant annual water mass variations of roughly 7 cm amplitude peaking in May and a semi-annual signal of roughly 3 cm peaking in June and in December. The amplitude of the annual water mass signal varies significantly from year to year and is significantly larger during 2004–2006 than in 2003 and 2007. This is also in agreement with the steric corrected altimetry.     

不同模型的地表质量异常一阶项、二阶项估计

为了合理补充重力场恢复与气候试验卫星（Gravity Recovery and Climate Experiment,GRACE）时变重力场的一阶斯托克斯系数（C₁₀、C₁₁、S₁₁）和替换二阶斯托克斯系数（C₂₀）,介绍了相关GRACE-OBP算法及其改进的算法,比较了相应的Chamber Model和4个Sun Model的一阶系数及其计算的地表质量异常,同时比较了基于卫星激光测距观测的Cheng Model与4个Sun Model的C₂₀及其地表质量异常。结果表明,GRACE-OBP算法的一阶系数、卫星激光测距观测的C₂₀及其地表质量异常与改进的GRACE-OBP算法在趋势项上有很大差异,但周年项差异相对较小。利用不同截断阶数和不同机构的GRACE时变重力场模型,对其趋势项和周年项都有一定影响,且对趋势项影响更大。因此,在计算陆地水储量变化时,建议使用改进的GRACE-OBP算法的估计结果,使用较理想的、截断阶数较高的GRACE时变重力模型。     

IGS Tide Gauge Benchmark Monitoring Pilot Project (TIGA): scientific benefits

The establishment of a long-term stable global reference frame is important for studying sea level records for, e.g., climate-related studies. GPS stations connected to the tide gauge benchmarks provide the necessary technique. However, the analysis of existing GPS solutions showed inconsistencies within the time series especially for the height component. To solve related issues, in 2001 the IGS Tide Gauge Benchmark Monitoring Pilot Project was established. The aim is the processing and re-processing of GPS data of stations at or near tide gauges in order to provide homogeneous and high-quality estimates of the vertical motion. A second objective is the establishment, maintenance and expansion of existing network of GPS stations at tide gauges. During the recent years six different analysis centers have processed overlapping GPS at tide gauge networks and are providing individual solutions allowing now to provide a combined solution. The ansatz for the combination is explained and quality measures are given. In addition, on the basis of the reconstruction of sea level anomalies, the benefit of using the combined TIGA solution is demonstrated.